On Twitter, Khalifa, a breakout hip-hop star from Pittsburgh, Pennsylvania, called the shooting "a senseless tragedy" in a tweet Saturday afternoon.

"Violence is never the answer," Khalifa wrote. "My prayers go out to the victim's family."

Police described the shooting suspect as a black man in his 20s with short hair who is between 5 feet 6 inches and 5 feet 8 inches tall. He was wearing a black shirt, a red baseball cap and possibly white pants, police said. The shooting reportedly happened backstage.

"We heard gunshots right as we were walking out to the parking lot," Cooper said. "We saw police officers running" toward the backstage area.

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Police believe the weapon used in the shooting was a black pistol with a large magazine, Tanaka said.

The motive for the shooting remained under investigation Saturday. Police turned to social networks, particularly Twitter, to solicit tips. They asked people to either call police or text information anonymously.

Khalifa played to some 22,000 people Friday night. The performance was part of his ongoing Under the Influence of Music Tour -- a not-so-subtle reference to Khalifa's reportedly prodigious use of marijuana.

In a statement, Live Nation, which organizes events at Shoreline, directed inquiries about the incident to Mountain View police. "We continue to cooperate with authorities in their investigation into last night's isolated incident," the company said Saturday.

The company said that Saturday night's Shoreline concert, featuring Rascal Flats, Sheryl Crow and Gloriana, would go on as scheduled. But the Khalifa concert scheduled for Saturday night in Chula Vista, near San Diego, was canceled and won't be rescheduled "due to an active investigation" of the shooting at Shoreline, Live Nation said in a separate statement. On his Twitter account, Khalifa blamed the cancellation on "a situation out of our control."

Tanaka did not immediately respond when asked if police had questioned Khalifa about the shooting.

Meanwhile, Khalifa's Sunday night concert in Irvine is still on the schedule, said Aaron Siuda, a Live Nation vice president in San Francisco.

Anyone with information, video or photos about the shooting is asked to call police at 650-903-6395 -- or email police@mountainview.gov. Anonymous text messages may be sent to 274637 with the code MVTips in the text.

Police are also asking people to use the hashtag #MVPDInfo in their Twitter and Instagram posts about the shooting so that they can examine relevant images and video.